You probably only want to add a couple more fish in that tank. Any more and it could get overcrowded. And I'd wait until you get some more rock added too. The rock provides a haven for the bacteria to live that break down the fish poop, and you already need more rock. 1 lb per gallon is a rough rule of thumb.
After you add the rock, give the bacteria a week or so to grow before adding more fish, or you could cause an ammonia spike that could harm or kill your fish.
When you are ready for new fish, a couple of clowns are always a popular choice, and they'll hold their own against the wrasses. Another good choice would be a diamond goby -- he'd really like sifting through by that fine sand you have and they are fun to watch dig. A damsel or chromis would also stand up to the wrasses. Avoid firefish or mandarins as they'll get bullied to death.
You don't have room for a tang or angel, so avoid those.